As is well known, the X-ray tomographic apparatus is used for forming a tomographic image of an object in such a manner that a number of projection data are collected by applying X-rays at various angles and the collected data are processed for image reconstruction. The data processing by such X-ray tomographic apparatus basically comprises the steps of preprocessing, reconstruction and postprocessing. In the preprocessing step, logarithmic conversions, predetermined corrections including X-ray intensity correction, beam quality hardening correction, scattering correction and etc., and filtering are performed and in the reconstruction step, the operation of back projections is performed by using the preprocessed projection data thereby forming image data indicative of a tomographic image of an object. The image data thus obtained are corrected by the postprocessing step to a form suitable for them to be stored in a recording medium.
As an apparatus for performing the above data processing steps at high speeds, there is employed a data processing unit comprising a combination of a fast processing unit (hereinafter referred to as "FPU") and one or a plurality of back projection units (hereinafter referred to as "BPU"). The FPU is provided with a main memorY for storing projection data and an operation part for preprocessing the projection data, and the BPU is provided with a computing element and an image memory having addresses each corresponding to each of pixels of a reconstruction image. To increase the image reconstruction speed, it is considered to operate a plurality of such data processing units in parallel relationships with one another but in that case, the following problems arise. That is, the data on the reconstruction image must be a total sum of back projection data obtained by each BPU and further, the reconstruction image data must be postprocessed by a desired FPU so that buses for reception and transmission of the back projection data between the BPUs and between the FPUs and BPUs. Consequently, the structure of the data processing unit becomes complicated.